Just finished putting in new VDO electrical temp, pressure and voltage gauges. I had installed a VDO tach a few months ago and it's been working great. Seeing as the car previously only had idiot lights I now have a whole new set of things to obsess over. I took the car out for a ride and everything worked fine...mostly. Temp stayed in the 190-200 range. Oil pressure was good at 40-50psi at 2000-2500 rpms and 20psi at idle after warming up. Voltage was reading around 14.5-15V at idle (headlights ON didn't seem to drop it at all and blower motor doesn't work so I couldn't try that), but seemed to go to 16V+ with any throttle. I wasn't too worried at the time. Then I decided to stomp on the throttle a few times to actually enjoy myself when I noticed that the gauges were all dead. I drove home and turned the car off then checked the fuses. Sure enough the 10A radio fuse was toast. With a DMM I also verified the voltage at the battery was 12.8V with engine off and 14.3V when at idle (which seems to match the gauge). I couldn't rev the engine and measure and I didn't think to check at the alternator. Replaced the fuse and cranked it over. Now the temp light was on and gauge read 230ish after just a few minutes of heat soak. Dropped down to 220 within a few seconds and light went out. The new VDO senders have dual output that allowed me to keep the idiot lights intact. The trip point for temp is 230. I think the stock temp switch is 240+ so that would explain why I never saw the light before (or the old sender was toast). A little coolant was dripping from the radiator overflow tube (by the neck). I'm sure it's been doing this since I've owned it but could never see it happen without a gauge and i aasume this is normal to have the temp rise right after the engine is turned off. All gauges and tach were working fine at idle and part throttle, but again a quick push to the floor and the 10A radio fuse popped again. I have all gauges' power tapped into connector that extends from ACC on the fuse block (gray wire from center of block in picture below). You can see my two spade terminals in the top right corner. Grounding is done through 14 AWG wire to one of the metal brackets under the dash. - Should I not be tapping IGN from the ACC port? - I'm guessing the volt gauge is correct as it matched the DMM so I guess I should be concerned about it getting up to 16V (at least)? - New voltage regulator? Alternator? - Anything I should check before/during poking around with a DMM?
Fuses blow when the amperage exceeds the fuse rating. This is usually from a short, and that can be from bare wire somewhere. You say the fuse blows when you romp on it. Think what else is on that fuse. The transmission downshift switch. Look at the wiring to the gas pedal switch, and down to the transmission. You may find the problem there. Something is causing that fuse to blow.
Ahh, thanks for that tidbit. I didn't see the kickdown switch on that circuit in the schematic I have. I'll have to look at that switch for sure. The radio isn't a part of the issue since it's not installed at the moment. I actually couldn't find the ACC terminal on the schematic either so I was a bit surprised to see the radio fuse blowing. I'm also not sure if the voltage going above 16 is playing a part. I know I didn't have any issues with the fuse, even with the tach, until I added the three new gauges today so it could just be that they draw enough current to put it over the edge when the kickdown switch is engaged. And now that you mention it I'm not entirely sure the transmission dropped into second when I floored it. Is there another convenient spade terminal I can tap off of on a different circuit for debugging? I really don't want to splice into a wire right now if I don't have to.
Might be something you did when installing the gauges. The gauges don't draw that much. You can tap into any of the fuses that are hot when the ignition is on. If your car does not have power windows, you can wire it right to the spot in the fuse block where the circuit breaker plugs into (center of the fuse block). If the voltage is going over 14.6 volts, you have a regulator problem. Take care of that first. I converted over to the internally regulated alternator on my car. Much less trouble, more reliable, and puts out much more at lower engine speeds. I'd look at all the wiring to the gas pedal switch, and at the transmission. If the wire is disconnected at the transmission and is laying on a metal part like the frame, every time you activate the gas pedal switch, you are shorting directly to ground. Maybe this might be coincidental to your gauge install, who knows.
Good points to look at. I know I knocked the connector off the gas pedal switch at least once when I was fishing the sender wires through the firewall. Maybe I did something to it putting it back on. Definitely need to look into it more. Now I'm off to find a regulator...and I know how much luck people have had with them lately.
A new regulator seems to have fixed the over voltage issue. It's now steady at 14.2 when at idle or full throttle. Now I have a no GEN light when the key is in accessory and a dim (about half brightness) GEN light when key is in IGN but not started. Light goes out when engine is running. Good enough for now. The fuse blowing when the kickdown switch is activated is still going on. I disconnected all gauges from the circuit to test. Turned the key to IGN, manually pushed the switch and poof there goes the fuse. I pulled the connector and just to be thorough made sure the switch contacts were closing as expected when depressed. The connector has two yellow wires in one side and a black with white stripe in the other. Verified with the DMM that one of the yellow wires goes to the radio connector. Strangely, a continuity test showed the radio connector also was connected to the black/white wire. That didn't sound right. Decided to measure resistance between the two terminals on the connector for sanity's sake and got a reading of 26 ohms. That still doesn't seem right. I thought the pedal switch was supposed to send 12V to the solenoid when pressed to the floor by closing the connection between the two terminals so there should be essentially infinite resistance between yellow and black until the switch closes. Then I see that my schematic says there is a jumper between the terminals. Not sure what that's supposed to be and if that's why I'm seeing some connectivity between the terminals. I also noticed that the schematic shows the other side of the solenoid tied to IGN via the backup light circuit. Not sure how it's supposed to switch if it's got 12V on both sides when the pedal is floored. Checked the connector on the trans and it seems well seated. Didn't have the foresight to pull the connector and ohm everything out back to the pedal switch until I got done with a test ride. By that point the exhaust pipes were ungodly hot and sitting close enough to the trans that I wasn't about to stick my hand up there and try to pry off the connector. That will have to wait until tomorrow. I'm really confused now. Need to check a different wiring diagram to see if the one I've been looking at makes sense.
Scott, See if you can get a click out of the downshift solenoid (inside the trans pan) by applying 12 volts directly to the connection on the transmission. You may have a problem with the solenoid, or the wiring inside the pan from the connection to the solenoid.
Thanks for the idea Larry. It's a good one and easy to check. Hopefully I get home in time to try that tonight. Things are certainly pointing that way and I'm not looking forward to changing it out seeing as the trans was rebuilt last year. ray: The connector at the trans has a pink and a black/white wire. The black/white comes form the pedal switch and it looks like the pink is from another switched IGN source. I assume I would need to have the key in the IGN position if the solenoid requires 12V on both pink and black/white. That's the part that is confusing me right now. That's not how the solenoids I'm used to work. Unless the pink wire is going somewhere else.
On your garden variety THM400, there is only one spade used for the kick down. That would be the vertical one. The horizontal one was commonly used for a pressure switch on the valve body. That pressure switch was a signal for the Transmission Controlled Spark emissions system, and unrelated to the kick down function. On a switch pitch transmission, that horizontal spade was the stator control. The downshift solenoid grounds to the case, and it has a single insulated wire lead that goes to the case connector. It is common for the insulation to deteriorate from the hot oil, and you may find that it is shorting to the case.
Well that would explain things. The wiring diagram I've been referencing shows both wires tied to the solenoid. That's what has been confusing me. It's obvious that the black/white one is tied to the pedal switch so I'll stick to that for now.
Larry, Thanks for the advice. It's definitely something at the connector on the trans or in the trans. I didn't have time to run a jumper to get 12V directly at the connector but I pulled it off to do some testing. With the connector removed from the trans I measure an open circuit between the pedal connector and ground like it should. When the pedal switch is depressed with IGN on the fuse is fine. Put the connector back on at the trans and the fuse will blow. Certainly looks like your thoughts that it's the solenoid or its wiring shorting out are spot on. I'll still try powering the solenoid directly as a final verification but for now the connector is pulled from the pedal and I live without a kickdown. How much fun is changing the solenoid and should I go ahead and swap the pressure switch at the same time since its wiring is probably just as bad? There was a drop or two of trans fluid in the connector when I pulled it so I may change that out too if it can be done from underneath. Otherwise I can live with the whole situation until I get around to restoring the car. Crappy thing is the trans was rebuilt a year ago. I guess I should have made sure the internal wiring was checked then.
Changing the solenoid is easy, 2 bolts and a connection and it is out, you just need to drop the pan. Might as well change the case connector also. The pressure switch is irrelevant unless you are still using the TCS system, and if you are, eliminate it. This isn't a concourse car, right? I would not even connect the pressure switch inside the transmission. Run one vacuum line from the carburetor to the vacuum advance, take out the thermovacuum switch on the intake, and the TCS is eliminated. http://www.transmissionpartsusa.com/TH400_TH425_transmission_case_connector_p/460-00046388a.htm http://www.transmissionpartsusa.com/SearchResults.asp?Search=detent+solenoid&Search.x=23&Search.y=8 http://transpartsdirect.com/th400-3l80detent-solenoid-8629935-p-10588.html
As usual, great advice Larry. I appreciate the help. The car definitely isn't concours but it's still mostly original unrestored. I'm keeping it in preservation mode for the time being until I decide whether to restore or keep it as is. Removing the TVS and and routing vacuum directly to the advance is on my list of fall projects. Is there any reason to disconnect the pressure switch wiring (or replace the switch itself) inside the trans while I've got the pan off. I figured it might be worth it just to avoid any other potential issues with old wiring while I'm at it. It's the original BB trans so I don't want to mess with it more than I have to. Thanks again.
No reason to mess with the switch or wiring unless you SEE a problem. It has nothing to do with the kick down. Either you have a bad solenoid, or the wiring inside is shorting. I would take the pan down and correct the problem. Then with the pan off, work the gas pedal switch with the Ignition on, and listen for the click, making sure it no longer blows the fuse.
Sooooo...what's wrong with this picture? This was my first peek into the trans while draining the fluid. The wire for the kickdown solenoid has a few cracks and one big chunk of insulation missing. No surprise since it's the original. That's not the problem. The problem is that whoever rebuilt the trans connected it to the wrong terminal on the case connector!! :shock: The pressure switch is connected to the vertical terminal which is tied to the kickdown switch. That obviously isn't going to work. Now I could swear the kickdown was working but that can't be the case. It never could have worked since it's on the horizontal terminal which goes to the TCS system. My memory must be crappier than I thought. Now I'm really wondering why the fuse just started blowing when I know very well that I've floored the car more times than I can count and it just started blowing. Did applying 12V to the pressure switch finally make it short permanently? I certainly wasn't applying 12V to the solenoid. Should I leave the pressure switch disconnected in the trans while I've got it opened up? Should I replace the pressure switch and just disconnect it up on the carb? o No: I'm replacing the solenoid regardless since I've got he pan off and the wire is all cracked. Just need to figure out what to do with the pressure switch before buttoning everything back up and adding fluid.
I would eliminate the TCS. Leave the pressure switch disconnected. The pressure switch goes to ground with the transmission in 3rd gear I believe, so if you floored the car, the gas pedal switch would ground out the lead. That is why it was blowing the fuse. The transmission can kick down without the gas pedal switch under certain conditions.
Well, I ended up putting everything back together before I read your post. Put in the new kickdown solenoid just to be safe and connected it to the proper terminal. Left the switch in place and connected it to the correct terminal also (still can't believe a trans shop got them backwards). I was thinking real hard about disconnecting it but didn't want to leave a wire hanging loose in the trans pan. Chassis manual says that it's supposed to be closed to ground until the trans is in reverse or third so it may not be fried though I kind of doubt it. Manual also says to test it make sure it doesn't get supplied more than 0.8A and knowing that it blew a 10A fuse a couple times makes me think it's probably dead. Oh well, I'm disabling TCS anyway. Of course, the obvious answer of just removing the wire completely didn't dawn on me until everything was bolted back up and running. ou: Then I started looking at just disconnecting the TCS solenoid at the carb and I see that someone has made some mods to the connector. Looks like there might be a resistor or something across the connector terminals. Didn't have time to tear into that yesterday so I left it as it was for now. Car runs great and trans seems to be working perfectly now. I'll play with the TCS and thermo switch later. Thanks for the guidance and sanity checks Larry. :TU:
