Hello Everyone,

My 1988 Volvo 760 Turbo Wagon has been an ongoing project for a good three years now. It's got tons of little electrical gimmicks but the one major thing that's inhibiting the operation of the car is the overdrive.

I understand that the electronic overdrive system and it's respected wiring is often faulty, and is a nightmare to trace and replace, and the knowledge I've gained of it thus far is that there is a switch, a relay and a solenoid. The switch puts power to the relay, and the relay flips on to engage the solenoid.

This is where I stand as of now: The overdrive works completely at random. After driving the car around for an extended period of time (took it to town and made several starts and stops at different places) when I was heading back on the final stretch of highway, O/D kicked in after I accelerated hard and let my foot of the gas a little to drop it down. I tried this again about 10 minutes later on an 80kph road and it nicely shifted into overdrive without a hitch. The next time I started it up, - nothing! I have since tried multiple times on trips of nearly equal length and speed to recreate overdrive, but the car refuses to shift. The overdrive switch works. I can press it once, and see the "disengaged" indicator, and press it another time and the indicator goes away followed by a barely audible click. If the relay were bad, would the overdrive disengaged light be on, or is that just signifying a bad connection to the overdrive relay on the back of the relay panel?

I took a look at the solenoid under the car, from where I can see the protective rubber boot on the top has cracked and weathered. I jiggled the wiring harness around for a bit but it did not seem to improve things after I took it out for a 15 minute highway drive.

I don't know where to start on this, is it the relay, the solenoid or the wiring between the two?

Any help would be appreciated, as I need to have the damn thing working by the end of next week so I can go on the hour long drive to the lake where my family reunion is being held!!

Thanks,
lobsterbox.

 

does the dashboard light reliably toggle when you push the button? when the light is on, the solenoid is off (and the transmission will stay out of OD), and when the light off, the solenoid is powered, and the tranny will shift into OD as it sees fit.

if the light is reliably toggling, then the problem is likely the solenoid itself, or the innards of the tranny (clogged pressure valves and such). or the wiring to the solenoid.


you can get a metal plate thing to replace the solenoid from IPDusa, which will leave it always in automatic OD mode, bypassing the relay and switch. you also can cut a notch on the tip of the solenoid that does the same thing. either of these fixes means you can't lock the tranny into 3rd for towing or going up/down mountain grades.

 

Thanks a million for your reply Pierce!

I might just go the route of replacing the solenoid and it's respective wiring harness. It's expensive, but it sounds like it might do the trick if the relay is functioning properly after all.

*EDIT* just a quick question here, is it possible to bench test the solenoid by applying a 12 volt current from a battery cable and jump leads?

Also, I wanted to apologize for posting a question about the 760 in the 740 forum, I didn't do it deliberately!!

 

the 760 turbo has the same engine as all the other cars in this forum, so it kinda belongs here.


I dunno, but I think if I was having problems with mine, I might just run a pair of jumper wires from the solenoid's wiring under the car to a LED or something in the car, so I could see if it was on steady when I was driving.

it *IS* possible the relay could be working fine for the dashboard light but the contact for the solenoid is bad, (or maybe the terminal soldering in the relay). and yes, 12V across the solenoid should engage it. I think one common failure mode of the solenoid is for its orifice to be clogged up with crud, this is more likely to happen on a tranny that hasn't had regular ATF flushes.

 

I would check the relay first. Mainly because its easy to access and you can pop the top off of it and re-flow the solder joints. This is a common failure point on these cars. Vibration over the years cracks the solder joints on the relays and then they make intermittent connections. Re-flowing them costs no money to try.

Good luck.

 

I will run some tests on the relay and the solenoid, thanks a lot for your advice guys!

On a tangent of sorts, I was hoping that someone may be able to answer some other questions, even though this is a thread titled "overdrive nightmare" perhaps starting a completely different one would not be entirely a good idea.

First one's about the climate control:
Last winter, the car was stored outside and rarely driven. The few times I started it up to move it to a different spot and whatnot, I noticed that the climate control was not giving me anything but defrost (not controlling the baffles properly) then after functioning like that for a while, it up and quit on me. I get no lights and no response when I turn the climate control on, nothing except a little hot air leeching out of the vents after driving the car for a while. The fuse is good, but I heard once about a climate control relay (I keep thinking it's the brown one next to overdrive) could this be causing the issue? Might someone here know how to remove the climate control module from where it sits behind the dash?

Second one is about the wipers:
The intermittent function does not work (instead the wipers are just on low), and the "single sweep" position won't shut off the wipers (they just continue to work on low.) You have to kind of wedge the switch between the single sweep and low positions to get it to stop, there has been a couple of times where I have gotten out of the car and slammed the door, and that stopped the wipers, which makes me think there might be a control module of sorts behind the dashboard, a loose connection maybe?

Third question is in regards to the sunroof not working:
Fuse is good, the previous owner said he replaced the motor twice (thinking that the first one was no good) I've read that on newer volvos you can clear the "upper module" by locking and unlocking the doors three times in a row, but that does not seem to have worked in my case.

Again any help would be appreciated, I know I have long posts with a ton of different questions in them, but for me and my involvement as a troubleshooter on lawn and garden tractor forums, specifics help. To everyone who has helped me out thus far, thank you kindly, and for anyone who may have answers to the questions that I post here, thanks in advance.

lobsterbox.

 

the climate control system is heavily dependent on vacuum to actuate the various air flaps. on the turbos, there's a vacuum 'accumulator' thing since boost is negative vacuum. if the hoses or the one way valve are leaky, it won't be able to move the flaps.

I don't have the wiring books on a 88 760 (only 91+ 9xx stuff, and 2xx too). are your fuses behind the ashtray, or are they on the end of the dashboard by the drivers door? early 760s were wired like 740/940 but later ones were wired like the 960s, where they moved stuff around. if its the 740/940 style, then the relays are behind the fuses behind the ashtray, take out the panel or cubby or stereo module above the ashtray, that has the ciggie lighter, for better access. if its the 960 style, then the relays are on the passenger side of the tranny hump, remove the passenger side kick panel under the dash to access. owners manual probably has a relay diagram. but frankly, your wiper problem sounds more like the switch to me.. again, the early 760 and later 760/960 have different steering columns and stuff.

your sunroof problem could be mechanical in the sunroof mechanism rather than the motor, about the only way to get at the mechanisms is to remove the headliner, something I'm reluctant to do on old cars as old plastic tends to be fragile.

 

Automatic Over-Drive Solenoid Bypass Kit
I just bought an 88 240DL. The OD doesn't work. The dear selector is all jacked up, the OD button doesn't work. I got under the car and the wiring coming from the button is all frayed and it looks like the wire grounded against the transmission housing. I tried putting the 2 wires that go into the shifter **** together and nothing happened, Maybe the relay is fried. But I touched them against the shifter base and got sparks.... ? Instead of going through and paying all the money to fix that stuff. I'm buying the above part since it seems like it will work. I've never needed to turn the OD off any of my other vehicles so I shouldn't need it anyways....

 

if you don't drive in the mountains, and/or you don't tow heavy loads, yeah, you don't need the OD button, and you probably leave your car in "D" all the time. Me, I drive a fair amount on mountain roads, and downshift the transmission quite a bit on downgrades, around turns, plus I sometimes tow a 1500-2000 lb trailer, where you don't want to use OD at all, leave it in third even at freeway speeds. So I like mine working.

you don't even need that kit, there's instructions online for filing a notch or something in the end of the solenoid, which has the same effect as that kit.

p.s. you really should start a NEW thread, and not piggyback on an old one about a different car with unrelated problems

 

I notched my od solenoid cover with a dremel and it works fine. Pierce is right about leaving it automatic.
I had intermittent rear wiper and rear defrost trouble. Both were a result of the relays being filled up with water and rusty crud. Both were tucked under the driver's side near the floor behind and to the left of the brake.
I have an '88 240 Dl that the PO did a bunch of work on and quite often relays are not near where the schematics say they should be.

 

hah. on 240's, the relays often came from the FACTORY not quite where the books say they are supposed to be.

 

I have an '87 760 Turbo and the relays are behind the radio in the center console. For the climate control, I had a similar problem and it ended up to be a bad vacuum cap on the vacuum canister behind the front bumper (and next to the radiator and intercooler). I found it after hearing a hissing sound up front. The same thing happened with my '87 740T... I picked up another cap from the local auto parts store and it was a quick cheap fix! J.C.

 

I was looking for solutions to my overdrive issues and came across this thread. I posted this as an option for the person who posted this thread as I didn't see it mentioned in the previous comments. I was just throwing out an option that I found and was going to try. Not trying to take over the thread. My apologies.

 

well, the original thread was from over 2 months ago, so the poster has probably moved on.

 

Very true, or the poster could still be working on it. There was no follow up post saying it has been resolved.

 

I apologize for not following up. Basically what happened was I decided to dremel out the O/D solenoid and bypass the lockout. 4th Gear works fine but it's hard to keep the car in it, it seems to want to kick down into 3rd with the slightest increase in acceleration. As far as the wipers, I cracked open the switch and found out that the contact points had flat spots in them. I soldered over the contact points and then filed the solder down flat against the surface. The front wipers work as they should, although I am still having the same problem with the rear wiper.

I gave up on trying to diagnose the climate control. There was no audible hiss anywhere in the cab or under the hood signifying a vacuum leak. I am reasonably sure that there was in fact power going to the ECC unit in the first place because the panel would light up with the rest of the interior lights. Since the A/C and Recirculation buttons were not lighting up under any circumstances (not even blinking) I bit the bullet and ordered a remanufactured unit off of ebay for $100 plus shipping.

We'll see how this goes.
lobsterbox

 

re: your kicking down too soon, sign of an overly tight transmission kickdown(*) cable.



(*) its more than a kickdown cable, it modulates the shift control pressure in the transmission, and controls how hard the bands grab the gears.